The above-mentioned conventional optical transmitter has a circuit configuration shown in FIG. 7. In other words, as shown in FIG. 7, an input terminal 51 is connected to one terminal of a resistor 52, and the other terminal of the resistor 52 is connected to the base of an NPN bipolar transistor 53 for driving a light emitting device. The collector of the NPN bipolar transistor 53 is connected to the cathode of a light-emitting diode 54 for optical transmission, and the emitter of the NPN bipolar transistor 53 is connected to one terminal of a resistor 55. The anode of the light-emitting diode 54 is connected to a power supply terminal 56, and the other terminal of the resistor 55 is connected to a ground terminal 57.
In the above-mentioned optical transmitter, when an optical transmission input signal applied to the input terminal 51 becomes high, the NPN bipolar transistor 53 conducts, a current flows from the power supply terminal 56 to the light-emitting diode 54, and the light-emitting diode 54 emits light. In addition, when the optical transmission input signal applied to the input terminal 51 becomes low, the NPN bipolar transistor 53 shuts off, no current flows to the light-emitting diode 54, and the light-emitting diode 54 stops light emission.
In the case of this circuit configuration, when an optical transmission input signal having an amplitude VIN shown in the waveform (a) of FIG. 8, that is, the output signal from a system LSI, is input to the input terminal 51 of the optical transmitter, the optical output from the light-emitting diode 54 has the same waveform as that of the output signal from the system LSI as shown in the waveform (b) of FIG. 8.
However, in the case when the potential of the input terminal 51, that is, the optical transmission input signal, is fixed high as shown in the waveform (a) of FIG. 9 because of the software or the like of the system LSI, the NPN bipolar transistor 53 maintains conducting. As a result, the light-emitting diode 54 emits light continuously as shown in the waveform (b) of FIG. 9.
Hence, it has been pointed out that various problems, such as battery exhaustion and breakdown of the light-emitting diode 54, may occur in PDAs and cellular phones.
For the prevention of the above-mentioned problems, a countermeasure is taken by disposing a protection circuit in the preceding stage of the input terminal 51 of the optical transmitter in many cases.
This protection circuit is configured to measure the pulse width of the optical transmission input signal by using a timer or the like and to forcibly stop the optical transmission input signal when the pulse width exceeds a certain time width.
In the conventional circuit configuration shown in FIG. 7, as described above, in the case when the potential of the input terminal 51, that is, the optical transmission input signal, is fixed high because of the software or the like, the light-emitting diode 54 emits light continuously. As a result, various problems, such as battery exhaustion and breakdown of the light-emitting diode 54, are caused in PDAs and cellular phones.
Furthermore, the configuration wherein the protection circuit is disposed in the preceding stage of the input terminal 51 of the optical transmitter is complicated, thereby causing a problem of high cost.